This invention relates generally to apparatus for measuring time and more particularly to an electronic time interval unit.
Time interval units are generally known and comprise electronic apparatus wherein an unknown time interval is measured between a start and a stop signal input. Time interval units typically include three parts including a coarse counter, a start interpolator and a stop interpolator. Most conventional time interval units measure time with a resolution finer than the coarse clock period and their performance is dictated by the performance of the interpolators. The interpolators of conventional time interval units normally involve one of two basic techniques. The first is a charge-discharge technique wherein a capacitor is charged and discharged at a constant rate, while the second uses an offset frequency technique and is analogous to a vernier time measuring device similar to that utilized in mechanical measurement tools.
With respect to charge-discharge type of time interpolation, one must accurately detect a zero crossing of the discharge ramp; however, this is susceptible to perturbations on the ramp and sensing the zero crossover point. Furthermore, charge and discharge ramps typically include harmonic frequencies with fixed amplitude and phase relationships. Real electrical circuits have non-ideal frequency and phase responses which result in less than ideal ramp waveforms which deviate from the average straight line which is desired.
As to the offset frequency type of time interpolation unit, it involves the use of a triggered oscillator utilized together with a fixed clock signal generator. The triggered oscillator must always start with low time jitter and with a fixed phase value. This starting phase can be influenced by noise from the trigger or by cross talk from the main oscillator. The performances of the frequency type time interpolator can never be better than the time jitter in starting the oscillator.
Accordingly, each of these known prior art approaches have inherent limitations which limit their resolution and accuracy.